Amplifier output circuit



March 2, 1965 5, AMOS ETAL I 3,171,891

AMPLIFIER OUTPUT CIRCUIT Filed Dec. 15, 1960 INVENTORS Bernard AmosJesse M Lafzm NEY United States Patent 3,171,891 AMPLIFIER OUTPUTCIRCUIT Bernard Amos and Jesse H. Lofton, Batavia, N.Y.,

assignors to Sylvania Electric Products Inc., a corporation of DelawareFiled Dec. 15, 1960, Ser. No. 75,995 2 Claims. (Cl. 1791) This inventionrelates generally to multi-channel reproduction and more specifically toa stereo amplifier output circuit.

Public acceptance of multi-channel reproduction systems in the form oftwo channel stereo circuits has increased the interest of manufacturersin seeking truly realistic sound reproduction. At first stereoreproducers were limited to an output system which utilized two spacedaudio transducers or speaker systems, each of which was intended toreproduce the whole gamut of audio frequencies. Problems which arose inpart from transcription errors and in part from speaker system spacingspointed up the fact that reliance upon two spaced sound sourcessometimes produced a noticeable and undesirable sound void in the centerposition. One suggested prior art solution was to insert a relativelylow frequency transducer in the center position between the two extendedstereo transducers and feed this center transducer through low frequencyfilters from both channels. In such units, the center transducerreproduces all low frequencies, which apparently are relativelynondirectional, and the two side spaced transducers reproduce the midand high audio frequency portion of the reproduction range. Furtherdevelopment lead to the realization that the center transducer orspeaker, under some circumstances, should be allowed to reproduce somemid range frequency audio signals even though such frequencies arenormally considered more directional in character than the lower portionof the audio range.

Though complicated circuits have been developed for feeding mid and highrange audio frequencies to the center transducer in a multi-channeloutput circuit, it would be desirable to provide means for accomplishingthis result Without expensive amplifiers, filters or crossover networks.

Thus it is an object of this invention to eliminate the need forexpensive circuit arrangements for feeding blendable portions of the midand high frequency portions of the audio range to a center transducer ina multi-channel transducer array.

It is a further object of this invention to blend at least portions ofthe mid audio range from both channels through a center transducer in astereo transducer array, under control of the feedback circuits in bothchannels.

Briefly, at least one aspect of the invention comprises feeding thespaced transducers through a common current path into which is inserteda relatively low frequency transducer connected in parallel with aresistance capacitance controllable bypass circuit.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe accompanying drawing which shows a multi-channel output circuitutilizing a specific embodiment of our invention.

In the embodiment of the invention shown in the drawing, it may be seenthat tubes 12 and 14 each comprise a single-ended final amplificationstage of a separate channel in a stereo amplifier. Though single-endedoutputs are shown for each channel, it is to be understood thatpush-pull outputs or any other output configiration, which occurs to oneskilled in the art, may be used.

Thus in the upper channel, audio signals are fed from the penultimatestage, not shown, through a coupling capacitor 16 to the control grid ofthe output tube 12. Also in the lower channel audio signals are fed fromthe lower channel penultimate stage, not shown, through couplingcapacitor 18 to the control grid of tube 14. A grid leak resistor 20 maybe coupled between the control grid of tube 12 and ground, and grid leakresistor 22 may be coupled between the control grid 15 and ground.Though tubes 12 and 14 may each have individual selfbiasing cathodecircuits, we have shown a resistance 28 which is bypassed by capacitor30 as a common selfbiasing circuit in the cathode circuits of both tubes12 and 14. Other types of biasing circuits may be provided wheredesired.

Output transformer 34 which passes the output signal from the upperchannel comprises a primary winding 36 coupled in the output circuit oftube 12 and a secondary 38 which is coupled to the transducer circuitryto be described. Likewise output transformer 40 in the lower amplifierchannel comprises a primary winding 42 in the output circuit of tube 14,and a secondary winding 44 which is also coupled in the transducercircuit to be described.

In the transducer circuit, terminal 48 is coupled to one side oftransformer secondary 38 and one side of transformer secondary 44 toprovide a common current return path for transducer current flow. Theremaining terminal of transformer secondary 38 is coupled through amid-range transducer or speaker 50 to a given potential plane, shown inthe drawing as ground. Coupled across speaker 50 through capacitance 52is a high range transducer or tweeter type speaker 54. In similar mannerthe remaining terminal of transformer secondary 44 is coupled through amid range speaker 56 to ground and through capacitance 60 and high rangespeaker or tweeter 58 to ground.

Low range transducer or woofer '62 is coupled between ground and thecommon transformer secondary terminal 48. Coupled across low rangewoofer 62 is a variable blend control comprising a variable resistor 64and capacitor 66. v

In use, the speaker unit 68 comprising mid range speakers 50 and 54 ispositioned in spaced relationship with the speaker unit 70, comprisingmid-range speaker 56 and tweeter 58. In order to provide an acceptablestereo effect, minimum spacing between speaker units should be at least8 or 9 feet with a maximum spacing distance governed by the wattage ofthe amplifiers and the power handling capacity of the particularspeakers selected. Woofer 62 may be positioned between speaker units 68and 70.so as to be in the middle, i.e., substantially the same distancefrom each side speaker unit.

Speakers 50 and 56, which reproduce mid-range audio frequencies,preferably should be selected so as to have a relatively low impedanceto audio frequencies below 200 cycles per second. In commercialapplications this can be accomplished by selecting speakers having aresonant frequency above 200 cycles per second. Woofer 62 should beselected to reproduce the low frequency portion of the audio rangepreferably between 50 cycles per second or slightly lower andapproximately 400 cycles per second at the upper end of the range.Usually such speakers also will reproduce higher frequencies withreasonable efficiency. Capacitor 66 is selected to have a value whichwill provide relatively low impedance to mid range and high frequencyaudio signals, and variable resistance 64 should be capable of insertinga resistance value somewhat higher than the impedance of speaker 62 inthe mid range audio frequencies.

'68, but also from speaker 70.

as ablend control to allow the listener to either fill in V or take outmid-range audio outputv from speaker 62.

"by the appended claims.

r 3 p .s r In operation, variable resistor 64 functions as a blendcontrol, allowing the listener to either blend in or remove mid rangeaudio output from speaker 62. 3 In other words, considering the audiooutput from transformer 34, it can be seen that full range audio signalcurrent passes through speaker unit 68 to ground and then back to theother side of secondary 38 through common terminal 48. Current flow fromground to terminal 48- divides in two paths. Some of the current flowsthrough speaker 62, and some of the current flows through variableresistor 64 and capacitance 66. If variable resistor 64 is adjusted toinclude a relatively high resistance in the circuit between ground and.capacitance 66, the majority of the return current flow tends to gothrough and drive the a coil of speaker As variable resistor 64 isadjusted to decrease the resistance between ground and capacitance 66,more and more of this return current flow tends to bypass the coil ofspeaker 62 and flow toward terminal 48 through capacitance 66. Insimilar manner, audio output signals impressed on speaker unit 70 fromtransformer secondary 40,,flow back to the other side of transformersecondary 44 through terminal 48 after passing through the parallelcircuit arrangement of speaker 62 andvthe series circuit comprisingvariable resistor 64 and capacitor 66.

In other words, as variable resistance 64 is adjusted to increase theresistance between ground and capacitance 66, speaker 62 is driven by anincreased mid-range audio signal contribution, not only from speakerunit Thus, resistance 64 acts Though there is a slight change in themid-range and high frequency voltages impressedacross. speaker units 68and 70, as resistance 64 is adjusted it is extremely difficult for theear to detect any change in the output volume of these speakers andextremely easy to notice change in the output of speaker 62. p a 7 Feedback connections 72 and 74 which are taken from the upper sides oftransformer secondaries 38 and, 44 are negative feed back pathsconnected to earlier amplifier stages, not shown. It is to be noted thatfeed back path 72 utilizes a signal taken from across secondary 44 andspeaker 62 as well as speaker unit 70-. Likewise, feed back path 74utilizes a signal taken from across secondary 38 and speaker 62 as wellas speaker unit 68. Thus speaker units 68 and. 70 as well as speaker 62are in the 1 feed back loops so as to minimize output distortion. 7

While there has been shown and, described what is at 'present consideredthe preferred embodiment of the invention, it'will be obvious to thoseskilled in the art that various changes andmodifications maybe madetherein without departing from the invention as defined Having thusdisclosed our invention, weclaim; *1. In' a multi-channel amplifier thecombination comprising, a series coupled pair of similar speaker unitshaving an intermediate terminal, said speaker units pro output circuitcoupled in series and having a second intermediate terminal, meanscoupling said series connected output circuits in parallel with saidvseries connected speaker units, a third speaker providing audio signalsover the low and middle portion of the audio range, a variableresistance-capacitance impedance coupled across said third speaker toform a parallel circuit, said capacitance having a relatively lowimpedance for middle audio frequencies, and means coupling said lastmentioned parallel circuit between said two intermediate terminals.

2. In a two-channel amplifier'having a separate feed back loop in eachchannel the combination comprising a series coupled pair of audiotransducers having an intermediate terminal, said audio transducershaving a relatively low impedance to frequencies across the low end ofthe audio range, a first channel output circuitv and secondchannel-output circuit, said output circuits being coupled in series,and having a second intermediate terminal, means coupling said seriesconnected output circuits in parallelwith said series connected audiotransducers, means coupling said first channel output circuit in thefeed back loop of the first amplifier channel, means coupling saidsecond channel output circuit in the feed back loop of the secondamplifier channel, a third audio transducer'having a given impedanceproviding audio signals over the low and middleportion of the audiorange, a

variable resistance-capacitance impedance connected across said thirdaudio transducer to form a parallel over'the middle portion of the audiorange and means coupling said last mentioned parallel circuit between'said two intermediate'terminals.

References Cited in the file of this patent UNITED- STATES PATENTS OTHERREFERENCES 7 Psychoa'coustic Applied to Stereophonic ReproductionSystem, by Goldmark et al., Journal of the Audio Engineering Society,April 1959, vol. 7, No. 2, pages 72-74.

Adding the Third Channelj High Fidelity Magazine,

April 1959, pages 109, 125 and-126.

' .Trimensional Stereo Speaker System, by Victor Brociner, Audio, June1959, pages 21-23 and 74.

I 7 "Phantom Channel for'Stereo, by Herman Burstein,

Electronic Work, June1959, pages 46, 47 and 80.

. Extended Stereo System, Electronics'World, January 1960, pages-50,156, 157,

1. IN A MULTI-CHANNEL AMPLIFIER THE COMBINATION COMPRISING A SERIESCOUPLED PAIR OF SIMILAR SPEAKER UNITS HAVING AN INTERMEDIATE TERMINAL,SAID SPEAKER UNITS PROVIDING MID AND HIGH RANGE AUDIO REPRODUCTION ANDHAVING A RELATIVELY LOW IMPEDANCE TO FREQUENCIES ACROSS THE LOW END OFTHE AUDIO RANGE, A FIRST AND SECOND CHANNEL OUTPUT CIRCUIT COUPLED INSERIES AND HAVING A SECOND INTERMEDIATE TERMINAL, MEANS COUPLING SAIDSERIES CONNECTED OUTPUT CIRCUITS IN PARALLEL WITH SAID SERIES CONNECTEDSPEAKER UNITS, A THIRD SPEAKER PROVIDING AUDIO SIGNALS OVER THE LOW ANDMIDDLE PORTION OF THE AUDIO RANGE, A VARIABLE RESISTANCE-CAPACITANCEIMPEDANCE COUPLED ACROSS SAID THIRD SPEAKER TO FORM A PARALLEL CIRCUIT,SAID CAPACITANCE HAVING A RELATIVELY LOW IMPEDANCE FOR MIDDLE AUDIOFREQUENCIES, AND MEANS COUPLING SAID LAST MENTIONED PARALLEL CIRCUITBETWEEN SAID TWO INTERMEDIATE TERMINALS.